Fig. 1: Posterior distributions for the power-law amplitude A and spectral index γ of the putative gravitational wave background in the European Pulsar Timing Array (EPTA) data.
From: Reading signatures of supermassive binary black holes in pulsar timing array observations
The results of a fit to Hellings-Downs correlations are shown in the left panel (a, full 25-year data, \(({\mathrm{lg}}\,A,\gamma )=(-14.5{3}_{-0.27}^{+0.15},4.1{4}_{-0.26}^{+0.56})\)) and the middle panel (b, the 10-year subset of the data described in ref. 6, \(({\mathrm{lg}}\,A,\gamma )=(-14.3{7}_{-0.57}^{+0.17},3.3{8}_{-0.25}^{+1.50})\)). The results of a fit of only temporal correlations to the 10-year data are shown in the right panel (c, \(({\mathrm{lg}}\,A,\gamma )=(-14.5{2}_{-0.80}^{+0.15},4.2{0}_{-0.72}^{+1.32})\)). Dashed red contours correspond to the result using the standard pulsar noise priors, and the blue contours correspond to our improved model. The horizontal dashed line corresponds to the background from supermassive binary black holes inspiralling entirely due to gravitational wave emission (subject to cosmic variance). Our improved model results in a lower median-aposteriori strain amplitude of the background and mitigates tensions with γ = 13/3. In marginalised distributions, shaded areas correspond to 1σ credible levels. In joint distributions, an inner dark area corresponds to the 1σ level, and the outer lighter area corresponds to the 2σ level.
